The ammonia removal cycle for a submerged biofilter used in a recirculating eel culture system

Aquacultural Engineering 31 (2004) 17–30

The ammonia removal cycle for a submerged biofilter used in a recirculating eel culture system Kuo-Feng Tseng∗ , Kuo-Lin Wu Department of Aquaculture, National Taiwan Ocean University, Keelung 202, Taiwan Received 5 February 2003; accepted 28 December 2003

Abstract Ammonia removal by a submerged biofilter varied in a cyclic manner with the growth and detachment of biofilm on the medium. Maintaining an active biofilm in a biofilter is required and is usually achieved by backwashing. In order to predict the time to be washed a laboratory scale system was designed and constructed to investigate the effects of operating parameters—temperature, ammonia and suspended solids concentrations—on the performance of biofilters. The results indicated that the ammonia removal rate per unit surface area of medium (specific ammonia removal rate, SNR) increased initially at the start of the operation, until it reached a maximum value. It maintained this maximum removal rate over a period, and then decreased sharply thereafter. The experimental results were used to develop a series of equations describing the relationship between ammonia removals and operating conditions (initial ammonia removal rate, temperature, concentrations of ammonia and suspended solids in influent). These equations were used to predict the biofilter performance in a commercial recirculating eel culture system. The observed values were within the 90% confidence interval of the model. Thus, the model allows a filter designer to select a backwash frequency that will maintain a stable water quality in recirculating aquaculture systems. © 2004 Elsevier B.V. All rights reserved. Keywords: Biofilter; Maintenance; Ammonia removal; Performance model; Recirculating system; Eel culture

1. Introduction Ammonia is toxic to eel (Anguilla anguilla) and causes growth retardation and sometimes mortality (Sadler, 1981). Though submerged biofilters are commonly installed in recirculating eel culture systems as a means to control ammonia level (Paller, 1992; Tseng et al., ∗

Corresponding author. Tel.: +886-2-24622192x5218; fax: +886-2-24633150. E-mail address: [email protected] (K.-F. Tseng).

0144-8609/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaeng.2003.12.002

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1996) below a safe concentration of 0.12 mg NH3 -N/l (Sadler, 1981), information on the proper use of biofilters is lacking and studies on the biofilter maintenance are few. A biofilter is a fixed film reactor in which only the active biomass on filter medium is responsible for bio-oxidation of substrates, regardless the total biomass present in biofilm (Lazarova et al., 1994). Over time, biofilm grows thicker as a result of microbial populations built up on the medium. The specific ammonia removal rate (ammonia removal per unit medium surface area; SNR, mg/m2 -h) increases until maximum SNR value (SNRm) is achieved in a nitrification biofilm with a critical thickness of about 15–25 ␮m (Liu and Capdeville, 1996). Thereafter, SNR remains constant despite further growth of the biofilm (Liu and Capdeville, 1996). This phase persists until the medium is overloaded. Excessive growth of microorganisms creates an anaerobic environment in the biofilm so that the cells underneath are deprived of nutrients and oxygen, and begin to die. Dead cells decrease the attachment of biofilm to the medium surface and eventually, the biofilm sloughs off (Stewart, 1993). The ammonia removal efficiency consequently declines sharply. Shedding part of biofilm biomass at critical points is a strategy devised to keep a biofilm in its optimal operating conditions. In a biofilter, growth and nitrification rate of biofilm are affected by different variables, e.g. ammonia concentration (Groeneweg et al., 1994), organic loading (Hanaki et al., 1990; Okabe et al., 1996), dissolved oxygen (Cecen and Gonenc, 1992; Hao and Huang, 1996; Huang and Hao, 1996), pH value (Groeneweg et al., 1994; Tseng et al., 1996), temperature (Groeneweg et al., 1994), and salinity (Tseng et al., 1994). In an eel culture system, water temperature fluctuates with the atmospheric temperature while the concentrations of ammonia and suspended solids (attributed by residual feed and feces) increase with the growth of eels. If a mathematical model can be developed to describe the correlation of ammonia removal and important water quality parameters—temperature, ammonia concentration and suspended solids concentration, the real-time ammonia removal rate can then be calculated and the need of biofilter maintenance predicted. The objectives of this study were to investigate the effects of temperature, ammonia and suspended solids on the efficiency of ammonia removal by biofilter and to develop a user’s guide for effective operation of biofilters. Based on the results, several model equations were developed. The accuracy of these models was assessed and validated by experiments conducted in a commercial recirculating eel farm.

2. Materials and methods 2.1. Biofilter system A schematic diagram of the experimental system is shown in Fig. 1. The system consists of three biofilters (BF1, BF2, BF3) made of an upright polyvinyl chloride pipe (7.15 cm inner diameter, 220 cm high) with a capacity of 8.63 l. The inlet port is located at bottom and outlet port at the height of 215 cm. The pipes were packed with cylinder-net shape polyethylene medium (Bio-Net, Norddeutsche Seekabelwerke AG, Nordenham; specific surface area of 150 m2 /m3 ) up to the height of 205 cm. The overall surface area was estimated at 1.23 m2 per biofilter. Synthetic wastewater used in the experiment was a mixture

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BF2

BF1

TBN

19

BF3

Tap Water TAD P

TC

P TSS

P

P

P P

P

P TA1

TA2

TA3

Fig. 1. The experimental apparatus (biofilter system). BF: submerged biofilter, TA1–3 : storage tank for ammonium chloride solution, TC : temperature control tank, TSS : storage tank for suspended solids solution, TBN : storage tank for basal nutrient solution of synthetic wastewater, P: pump, TAD : aeration tank for dilution water.

of basal nutrient solution (composed of Na2 CO3 ·7H2 O 47.5 mg/l, MgSO4 ·2H2 O 4 mg/l, CaCl2 0.25 mg/l, FeCl3 0.05 mg/l, and KH2 PO4 68 mg/l), dilution water and suspended solids (powdered commercial eel feed; composed of crude protein 55.8%, crude fat 8.2%, carbohydrate 15.8%, water 9.5%, and ash 10.7%). These were prepared separately in designated storage tanks (TBN , TAD , TSS ). The solutions were then pumped (P; Master-Flex, Cole-Parmer Instrument Co., USA) to temperature control tank (TC ) and homogeneously mixed by consistent stirring. Water temperature was adjusted in TC using a built-in heat exchanger before water was transferred to the biofilters. Ammonia solutions of various concentrations (Table 1) were prepared in different tanks (TA1 , TA2 , TA3 ) and pumped to the wastewater diversion tubes before reaching the inlet ports of the respective biofilters. Waterflow through the biofilter was of 288 ml/min and the apparent hydraulic retention time in biofilter was 30 min. Table 1 Concentrations of suspended solids (SS) and total ammonia nitrogen (TAN) stocked in stock solution tanks TSS and TA1–3 at different test runs Test runs

Concentrations

TSS (3 mg SS/l test run) TSS (9 mg SS/l test run) TSS (15 mg SS/l test run) TA1 (0.6 mg TAN/l test run) TA2 (1.3 mg TAN/l test run) TA3 (2.0 mg TAN/l test run)

259 mg SS/l 778 mg SS/l 1296 mg SS/l 17.3 mg TAN/l 37.4 mg TAN/l 57.6 mg TAN/l

The flow rate was 10 ml/min for each solution.

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2.2. Experimental methods Preliminary runs were conducted before the experiment was initiated. According to the results of preliminary experiments (three cycles over time) and a field survey of commercial farms, the biofilm detachment occurred once the biofilm reached a critical thickness, and the TAN removal rate decreased sharply (more than 30% of the previous day) during 1 day, afterward the ammonia removal recovered gradually 2 days after crash of biofilter. Preceding the tests, biofilters were preconditioned to allow establishment of nitrifying bacteria on the medium. The biofilters were operated at 25 ◦ C with continuous flow of influent water containing 0.6 mg TAN/l in basal nutrient solution until the ammonia removal rate (NR) reached about 30%. No suspended solids were added to the influent at this stage. During the experiment, tanks TA1 , TA2 and TA3 were loaded with ammonia solutions of 17.3, 37.4, and 57.6 mg/l, respectively (Table 1). Suspended solids solutions at different loading concentrations (259, 778 and 1296 mg/l; Table 1) were used at a test temperature of 22 ◦ C in the first run. Successive runs were conducted at 27 and 32 ◦ C under the same experimental conditions. Waters sampled from influent and effluent of biofilters were analyzed every other day initially, then everyday when stable ammonia removal was observed. Total ammonia (TAN) was determined using phenate method, nitrite (NO2 − -N) with colorimetric method, nitrate (NO3 − -N) with cadmium reduction method, total chemical oxygen demand (TCOD) with reflux colorimetric method, alkalinity (Alk) by titration, and suspended solids (SS) were determined by filtration with filter paper (Whatman, GF/C), following the Standard Methods (APHA, 1995). Dissolved oxygen and pH were also monitored. Ammonia removal rate (NR, %) is defined as total ammonia reduced in percentage after passing through a biofilter:
 TANi − TANe NR = × 100% (1) TANi where TANi and TANe are the total ammonia nitrogen (TAN, mg/l) in the influent and effluent, respectively. The experimental run was terminated when NR value decline to less than 70% of NR value in the preceding day, as a result of biofilm collapse. Specific ammonia removal rate (mg TAN/m2 -day) denotes the amount of ammonia removed by biomass on a unit surface area of medium per day and is calculated as follows: SNR = [TANi − TANe ] ×

Q A

(2)

where Q is the flow rate of biofilter (l per day) and A the total surface area of plastic medium in biofilter (m2 ). The duration of ammonia removal cycle (Dc , day) was defined as the time before the biofilter collapse, which is when NR increased and maintained a stable condition. The Dc value was determined according to the variation of NR with time. The period (in days) during which the ammonia removal rate tends to get higher (Di ) indicates the number of days required to establish an actively working biofilm. During this period ammonia removal rate rises from initial level (I, %) to a maximum level (M, %). It

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is also reflected in the specific removal rate, which increases from a low initial value to the maximum (SNRm). After reaching SNRm, the biofilter experiences a period of stable ammonia removal (Ds ; defined as the duration for SNR equal or greater than 95% of SNRm). The Ds value was determined according to the variation of NR with time also, and they vary from case to case and an extended period is always expected. The Di value was calculated by subtract Ds value from Dc value. 2.3. Statistical analysis The effects of temperature, ammonia and suspended solids concentrations on ammonia removal of biofilter were investigated. The total test runs were 27. The multiple regression model (y = αAa Bb Cc ) was used to analyze the experimental data using SAS program (SAS, 1986). Models were developed to interpret the implications of temperature, ammonia and suspended solids concentration on ammonia removal efficiency. The strength of each of these factors exerted on the removal rate, time required to form maximum working biofilm and duration of this desired status were presented as a power to the concentration of that particular factor. For the comparison of data from field experiments and the predicted values by model for field conditions, the 90% confidence intervals for model were calculated according to the method described by Neter et al. (1990). 2.4. Verification of ammonia removal model for biofilter Two field tests were carried out, once in winter (water temperature 25 ◦ C) and the other in summer (water temperature 31.5 ◦ C). The biofilter set-up used in the previous laboratory study was relocated to an indoor recirculating tank of a commercial eel culture facility (Taiwan Fisheries Research Institute, Keelung, Taiwan). Biofilters were preconditioned in the laboratory before transferred to the test site. Samples were taken from influent and effluent waters for water quality analysis; ammonia removal rates over time were calculated and plotted to reveal the profile of relative ammonia removal efficiency. The values of SNRm, Di and Ds with 90% confidences intervals were calculated in accordance with the model equations derived in the first part of the study and the method described by Neter et al. (1990). The actual values observed in the tests and the calculated values were examined to substantiate the models.

3. Results 3.1. Ammonia removal profile Similar trend in the ammonia removal profile of the three BF were observed regardless of the experimental conditions used. In general, specific ammonia removal rate rises slowly but steadily (Di period) until it reached a stable state (Ds period) followed by an abrupt decrease, indicating the collapse of biofilm (Fig. 2).

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Ammonia removal rate (%)

(22, 3, 0.6)

(27, 9, 1.3)

(32, 15, 2.0)

100 80 60 40 20 0 0

5

10

15

20 25 Time (days)

30

35

40

Fig. 2. The ammonia removal rates (%) of biofilter varied over time (days) plotted from the data of three test runs selected from 27 test runs. (22, 3, 0.6) indicates the parameters of influent wastewater: temperature, 22 ◦ C; SS concentration, 3 mg/l; TAN concentration, 0.6 mg/l.

3.2. Effects of temperature, suspended solids and ammonia concentrations on the performance of biofilter The SNRm, Dc , Ds and Di values for each test run were shown in Table 2. In all cases larger SNRm values were attained at higher temperature in less time (Di value) and shorter period (Ds value) except for cases of SS 3 mg/l with TAN 0.6 mg/l, and SS 9 mg/l with TAN 0.6 mg/l. The results indicated that suspended solids affected biofilter performance in a way that SNRm and Ds values decreased with the increase of influent SS concentrations, while influent containing SS 9 mg/l had greater obstruction (larger Di value) than SS 3 and 15 mg/l in the process attaining peak ammonia removal efficiency. Multiple regression analysis was carried out to elucidate the correlation between SNRm, influent water temperature (Te ), SS and TAN, and the following equation was derived Table 2 Ammonia removal efficiency (SNRm, Di , Ds ) of biofilter operated under various influent water temperature, suspended solids (SS) and ammonia (TAN) concentrations SS (mg/l)

TAN (mg/l)

3 3 3 9 9 9 15 15 15

0.6 1.3 2.0 0.6 1.3 2.0 0.6 1.3 2.0

SNRm (mg TAN/m2 -day)

Di (days)

22 ◦ C

27 ◦ C

32 ◦ C

22 ◦ C

27 ◦ C

32 ◦ C

22 ◦ C

27 ◦ C

32 ◦ C

181.6 339.7 417.0 174.9 326.2 400.2 137.9 252.2 309.4

185.0 356.5 440.6 181.6 332.9 420.4 144.6 265.7 319.5

188.3 373.3 460.7 185.0 353.1 437.2 151.3 282.5 332.9

16 15 14 16 16 15 15 13 12

12 14 12 16 14 14 14 12 11

10 12 12 13 14 13 12 11 9

18 15 13 16 13 12 13 12 10

20 14 12 14 13 11 13 12 9

18 13 11 15 12 11 12 10 8

Ds (days)

SNRm: maximum specific ammonia removal rate; Di : duration of increasing ammonia removal rate; Ds : duration of stable ammonia removal rate.

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(R2 = 0.9504): SNRm = 165.75Te 0.228 SS−0.16 TAN0.71

(3)

Eq. (3) indicates that the maximum specific ammonia removal rates of biofilter are positively affected by temperature and TAN but negatively by suspended solids in an exponential manner. The same data points were used to formulate the equation for Ds as shown below (R2 = 0.8842): Ds = 47.16Te −0.28 SS−0.17 TAN−0.28

(4)

Eq. (4) demonstrates that the length of period a biofilter removes most efficiently (Ds ) is negatively correlated to temperature, concentrations of ammonia and suspended solids. In other words biofilm is likely to crash sooner under the operation condition with high temperature, high TAN and more suspended solids. Besides temperature, suspended solids and ammonia, the ammonia removal rate at the initial stage (I) also affected the length of Di . The linear equation below (R2 = 0.8745) shows at less time required to reach maximum rate of ammonia removal is expected by high starting removal rate (I) at the beginning of the test and by more suspended solids and more ammonia wastes in the water. All the factors influenced Di value   I Di = 23.51 − 18.94 (5) − 0.13Te − 0.02 SS − 0.08 TAN M 3.3. Variation of other water quality characteristics on biofilter performance During the operating period, the concentrations of nitrite-N (NO2 -N) also varied with time (Table 3), increasing to a maximum (NO2 -Nm) at Di , decreased gradually to a stable concentration (NO2 -Ns), and increased again when the biofilter collapsed (Fig. 3). Oxygen Table 3 Effect of water temperature, suspended solids (SS) and ammonia (TAN) concentration on the variation of nitrite nitrogen concentration (NO2 -Nm, Di , NO2 -Ns) in effluent of biofilter SS (mg/l)

TAN (mg/l)

NO2 -Nm (mg/l) 22 ◦ C

27 ◦ C

32 ◦ C

Di (days) 22 ◦ C

27 ◦ C

32 ◦ C

NO2 -Ns (mg/l) 22 ◦ C

27 ◦ C

32 ◦ C

3 3 3 9 9 9 15 15 15

0.6 1.3 2.0 0.6 1.3 2.0 0.6 1.3 2.0

0.16 0.38 0.50 0.17 0.42 0.56 0.21 0.40 0.51

0.13 0.36 0.43 0.19 0.44 0.58 0.22 0.45 0.54

0.16 0.43 0.54 0.20 0.48 0.55 0.24 0.52 0.55

10 12 14 12 14 16 13 12 12

8 8 10 12 12 14 14 12 12

8 10 10 10 10 12 12 12 10

0.06 0.13 0.18 0.05 0.15 0.25 0.07 0.23 0.27

0.04 0.11 0.17 0.05 0.15 0.20 0.07 0.23 0.29

0.05 0.09 0.12 0.05 0.11 0.15 0.07 0.26 0.28

NO2 -Nm: maximum nitrite nitrogen concentration; Di : duration of increasing nitrite nitrogen concentration; NO2 -Ns: a lowest or stable nitrite nitrogen concentration.

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TAN

NO2-N

NO3-N

Total-N

1.4 N-cpds concen. (mg/l)

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

5

10

15 Time (days)

20

25

30

Fig. 3. The variation of nitrogen compounds (TAN, NO2 -N, NO3 -N; mg/l) and total nitrogen compound (Total-N, mg/l) concentrations in effluent of biofilter operated at 27 ◦ C, 9 mg SS/l and 1.3 mg TAN/l.

consumed by organic matter, measured as total chemical oxygen demand, was higher in the presence of more suspended solids. TCOD removals were 0.8, 1.3 and 4.1 mg/l in water containing 3, 9 and 15 mg SS/l in influent, respectively. The total alkalinity (as CaCO3 ) was quite low, about 45–55 mg/l, in the influent and decreased after passing through biofilter. The average alkalinity consumption per milligram of TAN removed was about 7.3–7.7 mg. Dissolved oxygen in influent was found in a narrow range of 7.1–7.8 mg/l. 3.4. Verification of ammonia removal model Water quality parameters in eel tanks prior to the start of filtration by biofilter are shown in Table 4. The profiles of ammonia removal by biofilters in the field tests over two periods are shown in Fig. 4. The predicted SNRm values of 95.8 and 108.7 mg/m2 -day were considerably higher than the observed values of 57.2 and 70.6 mg/m2 -day (Table 5). The time to build actively Table 4 Water quality (mean ± S.E.) of influent to biofilters operated in the eel culture farm Water parameters

Test 1 (winter)

Test 2 (summer)

(◦ C)

25.0 ± 0.2 0.22 ± 0.09 0.05 ± 0.03 148.1 ± 2.8 3.1 ± 0.4 14.8 ± 0.3 81.6 ± 2.5 7.30 ± 0.12 7.28 ± 0.13

31.5 ± 0.3 0.25 ± 0.08 0.05 ± 0.04 35.4 ± 2.1 3.4 ± 0.4 15.7 ± 0.5 87.9 ± 2.8 7.58 ± 0.15 7.05 ± 0.11

Temperature TAN (mg/l) NO2 − -N (mg/l) NO3 − -N (mg/l) SS (mg/l) TCOD (mg/l) Alkalinity (mg/l as CaCO3 ) pH DO (mg/l)

SNR (mg TAN/m2-day)

K.-F. Tseng, K.-L. Wu / Aquacultural Engineering 31 (2004) 17–30

80 70 60 50 40 30 20 10 0

FT1

0

5

10

25

FT2

15 20 25 Time (days)

30

35

40

Fig. 4. The specific ammonia removal rate (mg TAN/m2 -day) of biofilters varied over time in field test 1 (FT1) and test 2 (FT2).

working biomass was also less than the predicted ones. The duration of stable ammonia removal by biofilter during the actual field test was similar to the predicted values.

4. Discussion The recirculating eel culture system was introduced to Taiwan in 1992 (Shyu, 2001), and 17 farms were constructed. The original technology was developed by Danish Institute of Fishery Technology Association (DIFTA) in Denmark. The operator was advised to backwash the biofilter after about 30 days. The biofilter is composed of three cells, wherein one cell was backwashed at a time and the other two were operated under normal conditions to reduce the effect of decreasing TAN removal. 4.1. Ammonia removal cycle in a biofilter The nitrification process in a submerged biofilter, as quantitated by the ammonia removal rate (NR), follows a cycle that begins at a low baseline value, increases gradually over time until it reached the maximum NR, maintain a stable NR for a period, and then drops rapidly to the low starting level (Figs. 2 and 4). If a biofilter is used continuously without

Table 5 The observed values (O) from field tests and, the predicted values (P) and the respective 90% confidence interval values (CI) of SNRm, Di , Ds and Dc of biofilters calculated by ammonia removal model Field test 1

(mg/m2 -day)

SNRm Di (days) Ds (days) Dc (days)

Field test 2

O

P

CI

O

P

CI

57.2 11.0 25.0 36.0

95.8 13.5 24.0 37.5

46.9–194.7 0.6–26.4 13.5–42.7 14.1–69.1

70.6 10.0 21.0 31.0

108.7 12.7 21.4 34.1

54.3–218.4 0.1–25.3 12.1–37.5 12.3–62.8

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backwashing, the NR will fluctuate. The duration of NR cycle was found to range from 17 to 34 days in the present study. The fluctuation of ammonia removal is mainly attributed to the variation of the activities of nitrifying bacteria in the biofilm. As the biofilm thickens, the specific ammonia removal rate per unit surface area increases due to increase in biomass of nitrifying bacteria (Liu and Capdeville, 1996). The maximum specific activity of biofilm was found at certain film thickness for a nitrification biofilter (Liu and Capdeville, 1996). As the thickness of biofilm increases the substrates crossing through the biofilm will be limited, thus the bacteria in the inner layer may decay and die due to substrate limitation. 4.2. Effects of temperature, suspended solids and ammonia concentration on ammonia removal cycle of biofilter In the normal temperature range, the activity of microorganisms increases with temperature (Shammas, 1986). Nitrification rate expressed by the Monod equation has a linear relationship with temperature within a certain range (Randall and Buth, 1984). Wortman and Wheaton (1991) also found a linear relationship between ammonia removal rate and temperature at 7–35 ◦ C. In addition, increase of temperature causes a shift of ionized ammonia to the available unionized form (Groeneweg et al., 1994). Thus, an increase in temperature will result in higher ammonia removal and growth of nitrifying bacteria. This will decrease the time required for the biofilm to reach the critical active thickness (Di ) and the duration of ammonia removal cycle (Dc ). The specific growth rate of nitrifying bacteria (Paller, 1992) and the specific ammonia removal rate of biofilm increase with the increase of ammonia level (Zhu and Chen, 1999). It also decreases the Di and Ds values due to rapid biofilm growth, as observed in the present study. The effect of suspended solids on the other hand is more complex compared to the effects of temperature and ammonia on biofilter performance. The suspended solids may clog up pores in the biofilm and limit the diffusion of substrate (Sarner and Marklund, 1984). The organic matter dissolved from organic particulate will also promote the growth of heterotrophs, which will compete for dissolved oxygen and space with nitrifying bacteria (Sharma and Ahlert, 1977). Figueroa and Silverstein (1992) found that ammonia removal efficiency decreased with an increase in organic loading, either caused by particulate or dissolved organic matter. Furthermore, Golz et al. (1999) also detected that the major limitation on nitrification in floating-bead filter was the solids accumulation in filter. A higher organic carbon concentration caused by dissolved suspended solids will also increases the carbon/nitrogen (C:N) ratio in the water. High influent C:N ratio retards accumulation of nitrifying bacteria, especially NO2 -oxidizers, resulting in a considerably long start-up period for complete and stable nitrification (Okabe et al., 1996). In the present study, some of the suspended solids were adsorbed in biofilter. These particulate matters might limit the substrate transfer within biofilm and would likely be hydrolyzed and released as dissolved organic matter to favor the growth of heterotrophs and caused a decrease in SNRm value (Table 2). The biofilters with influent SS concentrations 3, 9 and 15 mg/l had average SS removal of 0.4, 0.7 and 2.1 mg/l, respectively. Based on the results of the present study, oxygen consumption did not increased significantly with

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Table 6 The influent dissolved oxygen (DOinf ) and minimum effluent dissolved oxygen (DOeff -min ) concentrations of biofilter, and maximum oxygen consumption (DOmax ) for water through biofilter operated under various temperature, influent suspended solids (SS) and total ammonia-N (TAN) concentrations DOmax (mg/l)

SS (mg/l)

TAN (mg/l)

DOinf (mg/l) 22 ◦ C

27 ◦ C

32 ◦ C

DOeff -min (mg/l) 22 ◦ C

27 ◦ C

32 ◦ C

22 ◦ C

27 ◦ C

32 ◦ C

3 3 3 9 9 9 15 15 15

0.6 1.3 2.0 0.6 1.3 2.0 0.6 1.3 2.0

7.85 7.85 7.85 7.84 7.84 7.84 7.81 7.81 7.81

7.24 7.24 7.24 7.28 7.28 7.28 7.29 7.29 7.29

7.08 7.08 7.08 7.10 7.10 7.10 7.07 7.07 7.07

5.33 3.24 2.22 5.34 3.18 2.15 5.75 4.09 3.21

4.70 2.42 1.28 4.62 2.44 1.26 5.15 3.32 2.55

4.48 1.97 0.79 4.41 1.98 0.84 4.80 2.91 2.14

2.52 4.61 5.63 2.50 4.66 5.69 2.06 3.72 4.60

2.54 4.82 5.96 2.66 4.84 6.02 2.14 3.97 4.74

2.60 5.11 6.29 2.69 5.12 6.26 2.27 4.16 4.93

the increase of SS concentration (Table 6) and therefore we suggest that the competition of heterotrophs was not serious and the effect of SS on ammonia removal was mostly caused by clogging. When the influent SS concentrations increased, the degree of clogging increased and the biofilm reached the critical active thickness in a shorter (Di ) duration (Table 2). 4.3. Application of ammonia removal model The specific ammonia removal rate increased more quickly and reached a higher maximum rate (SNRm) in field test 2. But, the Di , Ds and Dc values were less than those for field test 1 (Table 5). These results must be caused by a higher temperature (31.5 ◦ C) in field test 2 and can be indicated by model equations (3)–(5) (Fig. 5).

SNR(1)

SNR(2)

NO2(1)

NO2(2) 0.20

70 60

0.15

50 40 30

0.10

20

0.05

10 0

Effluent NO2-N (mg/l)

SNR (mg TAN/m2-day)

80

0.00 0

6

10

14 20 26 Time (days)

31

34

38

Fig. 5. The specific ammonia removal rate (mg TAN/m2 -day) and effluent nitrite concentration (NO2 -N, mg/l) of biofilters varied over time (days) for field test 1 (1) and test 2 (2).

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From the results obtained in this study, the model could predict the efficiency of ammonia removal for biofilter in a recirculating eel culture system. The procedures for applying this model are: (1) determining the initial ammonia removal rate (I, %), (2) determining the water temperature (Te , ◦ C), influent ammonia-N (TANi, mg/l) and SS concentrations (mg/l), (3) calculating SNRm (by Eq. (3)) and maximum ammonia removal rate (M (%) = (100% × A × SNRm)/(Q × TANi)), Ds (by Eq. (4)) and Di (by Eq. (5)) values, and (4) summing up Di and Ds to get Dc value. Biofilter should therefore be scheduled for backwash after a period of time based on the model. In order to maintain an even more stable water quality in the recirculating aquaculture system, the biofilter should be designed at least with three units in a system. Biofilters can take turn to be backwashed at various times. If the amount of biofilm removed from biofilter by backwashing is small, the initial ammonia removal rate of biofilter can still be high and the period of the operating cycle will be short. The mechanisms of nitrification in biofilter are very complicated and the performance of biofilter is difficult to predict by a simple model. In present study, the substrate diffusion affected by hydraulic condition was not included. Zhu and Chen (2001) indicated that hydraulic condition (expressed by Reynolds number) is an important factor limiting TAN removal rate. So, the application of the results of present study should be limited to the similar temperature, flow velocity through biofilter, freshwater and configuration of biofilter. In addition, when the composition of eel feeds change, the effect of suspended solids on the performance of biofilter may also change. In such case, the effect of suspended solids must be calibrated again.

5. Conclusions The ammonia removal rate of a submerged biofilter increases over time, reaches a maximum value, remains constant for a period of time and then decreases sharply. The ammonia removal cycle causes a variation of ammonia concentration in the effluent of biofilter. The duration of ammonia removal cycle of biofilter is affected by influent water temperature, and the concentrations of ammonia and suspended solids. The regression equations derived from the laboratory results have been validated in predicting the ammonia removal rate cycle of biofilter in the field tests at a commercial recirculating eel culture farm operated at similar conditions. The predicted time (minus 3–5 days) for biofilter to be backwashed can be calculated by applying the temperature, SS and TAN values to the equations. The biofilters in a recirculating system should be designed with multiple units, so backwashing of each unit at different time is possible to stabilize the total ammonia removal rate of the system thus had a more stable water quality. In considering the application of the model, the hydraulic condition and the ranges of temperature, suspended solids and ammonia concentrations must be checked. If the composition of feed differs significantly from present study, the effect of suspended solids on the performance of biofilter may be changed, and the coefficients of model equations must be calibrated before the model can be applied.

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Acknowledgements The authors would like to show their gratitude to the National Science Council of Taiwan, ROC (Project No.: NSC 89-2313-B-019-026) for providing grant to this study. The authors thank Dr. S. Miao and Dr. Y.H. Chen for their suggestions on data analysis, Dr. L. Zhu and Dr. E.M. Leaño for their suggestions on the manuscript.

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